1. Field of the Invention
This invention relates to coiled wire medical lead structures, such as transvenous, endocardial cardiac pacemaker and/or cardioverter/defibrillator leads. More particularly this invention relates to an improved structure for strengthening such leads to enable their intact removal by forceful traction after a period of chronic implant, specifically without the need of special lead removal devices.
2. Description of the Prior Art
Various types of transvenous pacing and cardioversion/defibrillation leads have been developed for endocardial introduction into different chambers of a patient's heart, typically the right ventricle or right atrial appendage, as well as the coronary sinus. These flexible leads usually are constructed having an outer polymeric sheath encasing one or more electrical, coiled wire conductors. One coiled wire conductor is typically attached at its distal tip to the shank portion of a tip electrode. In bipolar or multipolar leads, one or more further coiled wire conductors are provided in coaxial or co-linear relation to the first coiled wire conductor and are connected at its distal end to more proximally located, ring-shaped electrodes situated along the lead body. The proximal ends of each conductor are coupled to a connector which includes a single pin in unipolar leads and additional pins or in-line rings in bipolar and multi-polar leads.
The tip electrode is usually placed in contact with myocardial tissue by passage through a venous access, often the subclavian vein or one of its tributaries, which leads to the endocardial surface of the heart chambers. The tip electrode is held in place passively by trabeculations of myocardial tissue or actively through the use of an actively manipulated anchor or screw that penetrates the myocardium as described in U.S. Pat. Nos. 4,209,019 and 3,974,834, assigned to Medtronic, Inc. The distal ends of many available leads include flexible tines, flanges, or finger-like projections which extend radially outward and usually are molded from and are integral with the distal portion of the insulating sheath of the lead, usually proximal to the tip electrode and distal from any ring electrodes. These passive fixation mechanisms allow surrounding growth of tissue and scar in chronically implanted leads to fix the electrode tip in position in the heart and prevent dislodgement of the tip during the life of the lead.
In "acute" placement of the electrode tip, a blood clot forms about the fixation mechanism and insulating sheath (due to enzymes released as a result of irritation of the trabeculations of the myocardial tissue by the presence of the electrode tip) until scar tissue eventually forms, usually in three to six weeks. Until scar tissue develops, the fixation mechanisms described above prevent early dislodgement of the lead tip.
Although the state of the art in implanted pulse generator and endocardial lead technology has advanced considerably, endocardial leads nevertheless occasionally fail for a variety of reasons such as the following: insulation failure; sensor failure; coiled wire conductor fracture; and an increase in electrode resistance beyond a desirable level. Also, in some instances, it may be desirable to electronically stimulate different portions of the heart than are presently being stimulated with leads already in place. There are a considerable number of patients who have had one or more, and sometimes as many as four or five previously and currently used leads in their veins and heart.
The risks of leaving unusable leads in the heart and venous path include the following: an increased likelihood of infection; a potentially fatal complication which may necessitate removal of the lead; obstruction to blood flow as in "SVC syndrome"; and an increased likelihood of the formation of blood clots which may embolize to the lung and produce severe complications and even death. In addition, extra leads in the heart can interefere with cardiac valve and mechanical function. Thus, it is desirable to remove old unusable leads. Finally, the presence of unused leads in the venous pathway and inside the heart can cause considerable difficulty in the positioning and attachment of new endocardial leads in the heart.
In patients where implanted leads fails, it is desirable that they be removed. However, surgeons usually have avoided attempts to remove previously implanted leads because the risk of removing them exceeds the risk of leaving them in. Heretofore, removal techniques in the replacement surgery typically have involved applying traction to the old lead either by grasping the exposed proximal end of the lead and attempting to manually pull the lead out of the vein, or by attaching the proximal connector end to a line and weight suspended by a pulley and allowing the steady traction to gradually pull the lead free from the patient's heart over several hours to days, as herein shown in FIG. 2 and described in numerous published papers, such as "Incarceration of Transvenous Pacemaker Electrode. Removal By Traction," by A. M. Bilgutay, et al., American Heart Journal, Vol. 77, No. 3, pp. 377-379, March 1969.
Grasping and applying traction on the proximal ends of the chronically implanted leads results in directing pulling forces substantially along the length of the lead. These pulling forces are transmitted through the lead to its distal tip. Because of the fibrosis enveloping the electrode, substantial resistance to the pulling forces is experienced, and stress is placed on the lead as well as the heart.
As described above, endocardial lead construction typically includes a polymeric insulating sheath, within which one or more electrical, coiled wire conductors are mounted and attached to distally located electrodes and proximally located connector pins. Unfortunately, these leads have typically been constructed in such a manner which tends to make their subsequent removal difficult. When subjected to pulling forces along its length, such a lead usually disassembles. The polymeric insulating sheath can break away from the proximal and distal ends of the lead while the coiled wire conductor is stretched until it breaks or has to be cut off at the venous access site. The exposed end of a coiled wire conductor, once extended and stretched, may present the risk of cutting adjacent tissue if left in place. In such cases, only open heart surgery can fully remove the lead.
A further complication of applying direct manual pulling force to the proximal end of the lead is the avulsion of the heart, which can induce arrhythmias or even lead to death. Thus, care must be taken to observe the procedure under fluoroscopy and to avoid either breaking the lead structure or causing avulsion of the heart.
Various techniques and lead removal tools have been proposed to temporarily strengthen the lead body during the attempted removal and/or to cut away the connective tissue and, in some instances, the fixation mechanism, leaving part of it in the heart. Such tools as are disclosed in U.S. Pat. Nos. 4,988,347, 4,574,800, and 4,582,056 typically involve the use of a special stylet inserted into the lumen of the coiled wire conductor having an expandable member at its distal tip for wedging into the distal coiled wire conductor at its connection with the tip electrode shank and applying traction to the combined lead and attached wire stylet. Furthermore, it has been proposed to employ a catheter advanced over the outer sheath of the lead to sever the connective tissue adhering to the sheath along its length and at its distal tip. Other procedures include special grasping tools for grasping the lead body, as described, for example, in "Percutaneous Removal of Ventricular Pacemaker Electrodes Using a Dormier Basket," by C. J. Foster, et al., in Int. Jr. of Cardiology, 21 (1988) 127-134, and publications cited therein.
The procedures employing these removal tools are relatively complex and expensive and are usually resorted to only in those instances where the application of traction has proven ineffective. It is therefore desirable to provide a removable lead construction which reduces the necessity of resorting to the use of special tools or procedures.